Bless your heart! What a bittersweet position to find yourself in - finally able to leave a violent and abusive situation yourself, but carrying deep and lingering concern in your heart for those you love who must remain.
As you know, I am not trained as a therapist, and this seems to me to be a situation where help from a professional counselor would be very appropriate. Perhaps a pastor at your church or a social worker through your school system could get you connected to some services.
I would also recommend establishing a relationship with a counselor at the student health center of your college soon after you arrive. It would be helpful to have a rapport already established in case things really go south back at home while you are away. The feelings of powerlessness can be very tough to handle without some support. There may also be some emotions that come to the surface once you are finally somewhere safe, and you might like some help processing those.
Before we get into some ways you can help your sisters from a distance, I want to encourage you to consider telling a teacher, doctor, or other responsible adult about what's been going on in your home. I realize that telling doesn't always solve everything, but I'm concerned that you and your sisters are carrying such a heavy burden on your own when there may be help available. The best thing you might be able to do for them before you go is to get them some local, professional assistance.
That said, here are a few ways you can help your sisters when you are away. First, be a steadfast role model of healthy relating for them. Keep in touch by phone, text, or skype. Listen, show respect, express that you care, and inquire about the things that are important to them. Stay involved in their lives to the extent that you can do so without compromising your studies or detracting from your friendships at school. Be the contrast to the unhealthy relationship they have with your mom, so they know what else is possible. If things get scary or dangerous for them, don't hesitate to encourage them to tell a teacher or neighbor what's going on.
Second, listen, listen, and listen some more. Listening is our most powerful tool for helping to build emotional resilience. Research has shown that it only takes ONE supportive, accepting relationship in the life of a child to make a significant difference. You know what they are going through better than anyone, so you are primed for understanding and empathy. You don't have to offer a solution to make a difference. Listening compassionately and attentively while they vent their frustrations is a powerful act of support, and helps clear the static in their minds so they can think clearly again.
Since you are obviously going to be very busy with your own life, balance in this area will be a moving target - sometimes you will feel too connected to home, and sometimes you'll feel too far away. Try to tune in to your own needs as often as you can, and allow yourself to make adjustments in your involvement to keep yourself centered and engaged in whatever makes you happy.
You didn't mention the ages of your sisters, but before you leave, perhaps you could help them to cultivate some relationships with neighbors or parents of friends who would welcome them into their homes on very short notice when your mom is getting wound up. There may be people nearby who are willing and able to provide a safe place to hang out until she cools down. This can be informal - in my daughter's circle of friends it's understood without being said that some moms are more emotionally stable than others, and those homes are more likely to be the places the kids will gather.
Below are some links to articles by Patty Wipfler of www.handinhandparenting.org. Although they are mostly targeted at parents with children on the younger side, I find that it's not hard at all to adapt her suggestions for older kids or teens. The spirit of compassion that she recommends applies to all ages. I have a tremendous respect for her approach, and since she's already said it better than I ever could, I will refer you to her articles for some additional information and suggestions.
I hope this helps. Please keep in touch and let me know how thing are going for you.